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hardware.txt
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1996-07-14
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11KB
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287 lines
Hardware Information
--------------------
There are now two versions of Pasokon TV 3 which use different
types of hardware:
Pasokon TV Classic
------------------
The Pasokon TV Classic interface fits inside an expansion slot
of a computer. This design has many advantages over the
simple single chip circuits used by many other SSTV systems
- Built-in audio filter to dig signals out of the noise.
- Hardware decoding of SSTV signal for more consistent results.
- On-board crystal oscillator for precise timing.
- Does not tie up a serial port.
- Built-in relay for PTT and transmit audio switching.
E Z SSTV
--------
E Z SSTV uses a simple single op-amp circuit connected to a
serial port. If you are already using one of these, you can
use the same interface with little or no modification:
* AEA FAX
* EZ SSTV (version 2)
GSH-PC
JVFAX
MSCAN
ProSkan
* SSC PC SSTV
* SSTV Explorer
VESTER
* = Note: Reverse power must be selected for these.
This mentioned below and explained in more detail in
the file POWER.TXT or from the Help button of the
interface Power polarity dialog box.
These are the necessary connections for receiving:
Signal 25 pin 9 pin
Name connector connector Function
------ --------- --------- --------
DTR 20 4 + power for op amp
RTS 4 7 - power for op amp
GND 7 5 ground
DSR 6 6 amplified audio to computer
Interfaces marked with "*" in the earlier list use RTS for +
power instead of DTR. The software has an option for this.
Note that some interfaces use a full wave rectifier so it will
work with either polarity of RTS and DTR.
A Very Simple Receive Interface
-------------------------------
This is about as simple as you can get. The basic idea is to
amplify the received audio and feed it into one of the pins
of a serial port. The computer measures the time between the
zero crossings and decodes an image from the timing.
You might even be able to use an 8 ohm to 1000 ohm audio
transformer to increase the signal voltage sufficiently and
forget about the op amp. I haven't tried this yet.
A well stocked junk box probably has all of these. Otherwise,
you can get everything at your local Radio Shack store. The
resistor and capacitor values are not at all critical. Use
either a 25 or 9 pin connector depending on the type of serial
port available. Just about any op amp will work; we're just
using it as a limiter at audio frequencies so there is no
benefit to using something with super high speed.
part # $
------ -----
U1 741 or similar op amp 276-007 0.79
R1 47K resistor 271-1342 0.39
C1 0.01 uF capacitor 272-131 0.49
J1 DB25S 276-1548 1.99
or DB9S connector 276-1538 1.29
- Shielded cable for audio ? ?
There are many variations on this basic circuit. Others use
diodes for reverse voltage protection, capacitors for power
filtering, and/or transformers for breaking up ground loops.
This is trimmed to the bare essentials but still seems to work
well.
U1 - top view
(pins pointing away from you) 25 9
notch on this end J1 name pin pin
and/or dot near pin 1 ---- --- ---
Audio #----U----+ +-----+
from | 1 8 | +----|--O | DTR 20 4
Receiver | | | | | | |
-------| |---+--------| 2 7 |----+ | |
| | | | | | |
C1 | +----| 3 6 |---------|--O | DSR 6 6
< | | | | |
> | +--| 4 5 | | |
R1 < | | +---------+ | |
> | | | |
< | +----------------------|--O | RTS 4 7
> | | |
| | | |
------------+---+------------------------|--O | GND 7 5
Audio +-----+
Ground
Transmitting
------------
When transmitting, audio is available from both the PC's
speaker and the serial port.
Transmit audio from serial port
-------------------------------
Signal 25 pin 9 pin
Name connector connector Function
------ --------- --------- --------
TxD 2 3 transmit audio
RTS 4 7 + during xmit to drive
transistor for PTT
GND 7 5 ground
Advantages:
- No need to connect a wire inside computer.
- Safe. Can't harm serial port by shorting wires to ground
or each other.
- Can be used with laptops or other computers where it is
very difficult to get at the internal speaker.
Disadvantages:
- Tones are less accurate than from speaker. Transmitted
image quality will not be as good.
The signal from the serial port is about 20 volts peak to peak
so this must be attenuated quite a bit before applying it to
the transmitter. Use something like this:
Serial Port | |
TxD signal -----/\/\/\/-----+ +-------+
| | |
100k | 10k pot
0.1 uF <
>
<
> <-----------
<
> Audio
| to
| Transmitter
Ground ----------------+
Some designs also provide some low pass filtering to reduce
the harmonics present in the square wave signal. I don't know
if this is really necessary because the transmitter has
limited bandwidth anyhow and will round off the edges.
Transmit audio from internal speaker
------------------------------------
Advantages:
- Tones are more accurate than from serial port.
This results in much better quality transmitted images.
Disadvantages:
- Must connect a wire inside computer.
- You can damage the computer if this is done improperly
or the speaker wire is accidently shorted to ground.
CAUTION !!! v v v v v v v v v v
Be very careful if you decide to connect a wire
to the internal speaker. If the designers wanted you
to connect something to it, they would have provided
a jack.
In my computer (don't know about others) one side of
the speaker is connected directly to 5 volts. If you
accidentally short this to ground, the computer might
be damaged. I would strongly recommend that a 1k
resistor and a capacitor be used between the speaker
and the wire coming out of the computer. This will
limit current and prevent damage if the wire is shorted
to ground.
CAUTION !!! ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
You will have to experiment to see which side of the speaker
has stronger audio. Be careful!!! One side of the speaker
might be connected directly to 5 volts. Shorting this to
ground could cause extensive damage.
WARNING! Connect only a single wire to the speaker. DO NOT
connect a ground wire to the other side!!!
.
Inside computer . Outside
.
.
PC Internal | | .
Speaker -----/\/\/\/-----+ +-----------+
| | . |
1k | 10k pot
0.1 uF <
>
<
> <-----------
<
> Audio
| to
| Transmitter
Ground -----------------+
(not other side of speaker)
I would suggest placing the resistor and capacitor inside the
computer and insulate everthing well with heat-shrink tubing.
With the resistor and capacitor to limit current, you won't
have to worry about damaging the computer if the external
wire is accidentally shorted to ground.
Push To Talk (PTT)
------------------
There are 3 different ways to activate the transmitter when
sending an SSTV image:
- Manually.
- VOX activated by SSTV audio signal.
- Computer controlled PTT.
If you want the computer to control the transmitter with the
PTT line, use the following circuit. This should work with
most modern transceivers with low voltage, low current PTT
circuits. For older rigs you might have to use a relay to
handle higher voltage or current.
Xcvr
/---------- PTT
| / c
Serial |\ | b | /
Port RTS ----\/\/\/\---||>|-----------|< 2N2222
|/ | | \ or similar
10k | _| e NPN
1N4148 \
|
Ground
The diode is important because applying a negative voltage
to the base of the transistor might damage it.
Configuration File
------------------
The serial port number, and possibly other attributes, must be
specified in the PKTV.CFG file.
See CONFIG.TXT for more details of the COM_PORT option.